All industrial societies are faced with significant environmental problems associated with industrial waste materials, many of which are hazardous to both animal and plant life. Examples of such waste materials include sludges which settle as sedimentation layers at the bottom of the sea, lakes, and rivers; effluent sludges discharged from various industries including pharmaceutical, tanning, paper and pulp manufacturing, wool washing, fermenting, food processing, metal surface processing, plating, ore dressing, coal washing, and fume desulfurizing; as well as other wastes such as sewage sludges discharged from sewage processing stations, and those resulting from the refining of petroleum products. Such wastes are often contaminated with substances which can have an adverse effect on the ecological system. Contaminants found in such wastes often include unacceptable levels of heavy metals such as copper, lead, cadmium, arsenic, mercury, hexavalent chromium. Also found are other chronically toxic compounds such as PCB, PCP, DDT, 2-BHC, Dieldrin, Chlordecone, Mirex, Parathion, cyanic compounds, alkyl-mercury compounds, dioxins, furans; and the like. These waste materials are sometimes referred to herein as environmentally unacceptable.
The treatment and handling of such contaminated waste materials, many of which can be classified as hazardous, is strictly regulated by one or more governmental agencies because of their potential harm to the public welfare. As such, a great deal of work has been done in recent years in developing methods for safely handling these materials and for neutralizing their troublesome characteristics so they can be safely discarded.
Non-limiting examples of methods which have been developed and which have met with varying degrees of success, include sorption, adsorption, volatilization, biodegradation, chemisorption, passivation, ion-exchange, encapsulation, and embedment through solidification into a monolith structure, as well as stabilization of chemical constituents. Sorption involves adding a solid to the hazardous waste material to soak-up any liquid which is present. Non-biodegradable materials are typically used as the sorbent. Such materials include activated carbon, anhydrous sodium silicate, various forms of gypsum, celite, clays, bottom ash, fly ash, fly dust, kiln ash, and cement kiln dust. Biodegradable materials can also be used, such as peat moss, rice hulls, sawdust, and the like. These treatments primarily use biodegradation for reducing organic constituents of the waste material. The sorbent may interact chemically with the waste material, or it may simply be wetted by the liquid portion which is retained in the sorbent as a capillary liquid. Sorption generally cannot be used with many types of hazardous waste, such as those which contain potentially hazardous components, because they can often be easily leached from the sorbent. This is because sorption typically only requires that it be mixed with the waste. Further, as governmental regulations become ever stricter, sorption becomes less and less attractive for meeting such regulations.
Another method for treating hazardous waste involves the consolidation and solidification of the waste into a solid block of material that has relatively high structural integrity. The resulting block is often called a monolith. The monolith can be as small as the contents of a steel drum, or it can encompass the entire waste disposal site, called a monofill. The components of the monolith do not necessarily interact chemically with the reagents, but are usually mechanically locked within a solidified matrix, called microencapsulation. Contaminant loss is primarily limited by decreasing the surface area which is exposed to the environment and/or isolating the contaminants from environmental influences by encapsulating the waste particles. Wastes can also be microencapsulated. That is, bonded to, or surrounded by, an impervious coating.
While solidification can be very effective for treating some hazardous wastes, recently passed governmental regulations are placing greater demands on this technology. This is because of the ever stricter limitations relating to acceptable levels of leachates from the solidified block. Also, the block must withstand ever greater physical pressures without cracking and exposing contaminants to environmental influences.
Other methods of hazardous waste treatment include inorganic and organic stabilization. Unlike solidification processes which convert wastes into a solid mass, stabilization processes can reduce the solubility or chemical reactivity of the waste. Stabilization processes typically include adjusting pH, converting metals to hydroxides, and/or establishing oxidation-reduction conditions to prevent solubilization or leaching of contaminants into ground water. The most commonly used inorganic stabilization processes include mixing the waste product with inorganic materials such as fly ash, cement kiln dust, lime kiln dust, hydrated lime, Portland cement, or other pozzolanic materials. Stabilization processes, like solidification, are being pushed to meet the stricter governmental regulations.
A typical stabilization process is taught in U.S. Pat. Nos. 4,781,842 and 4,902,431 wherein a sewage sludge is stabilized and converted to fertilizer by mixing the sludge with an alkaline material which is sufficient to raise the pH to at least 12. The mixture is then allowed to dry for at least one day. The alkaline material is selected from cement, kiln dust, and lime dust, to achieve chemical stabilization. Bulking materials, such as slag fines, fly ash, gypsum, etc. may also be added. Such a process is primarily a drying process to eliminate offensive odors and pathogenic microorganisms. The process is not capable of generating a substantial amount of heat to destroy many of the contaminants.
Also, U.S. Pat. No. 4,859,367 teaches a waste solidification method wherein toxic mine tailings are incorporated into a cement mixture which contains a mineral binder selected from the class of alkali-activated silico-aluminate geopolymers which is stated to be related to natural and synthetic zeolites and feldspathoids.
Although a significant amount of work has already been done to treat contaminated waste materials, there is still a considerable need in the art for improved methods for treating and neutralizing such materials.